This project will examine the cognitive and neural mechanisms of visual selective attention in humans. Several experiments will assess the role that attention plays when human subjects perform visual discriminations, ranging from the detection of simple features such as color and orientation to the discrimination of complex conjunctions of form and color. These experiments will determine whether spatially focused attention is necessary for performing these discriminations, and the outcome of these experiments will have important implications for the computational role of selective attention. Additional experiments will explore the specific neural structures in which selective attention operates and the time-course of attentional allocation. The allocation of attention will be measured with a combination of behavioral measurements and electrophysiological recordings (using the event-related potential technique). The necessity of spatially focused attention for a given discrimination will be assessed by requiring subjects to perform two concurrent tasks: 1) a primary task that requires the subject to perform the discrimination of interest; and 2) a secondary task that will absorb any available attentional resources and ensure that subjects do not focus attention onto the primary task targets unless these targets cannot be discriminated without spatially focused attention. The neural structures in which attention operates will be assessed by measurements of electrophysiological responses with known neural generator sources. The time-course of attentional allocation will be measured by using behavioral and electrophysiological measures of attentional allocation to track the focusing of attention over time as subjects scan stimulus arrays. This research is relevant for several important mental health issues. Specifically, several psychiatric and neurological disturbances such as schizophrenia are characterized by deficits in attention, and an understanding of the function of attention in normal individuals is important for understanding and ameliorating these disorders. In addition, many developmental disorders such as dyslexia and attention deficit disorder also involve deficits in attentional processes. Reading, in particular, is extremely dependent on the mechanisms of attention that will be studied in this project, and the results of the proposed experiments may ultimately be useful for developing treatments for reading disabilities.